Applying glare to selected color only
The "correct" way to do it would be to have the material to emit more than 1 in value and set the threshold so that the rest gets cut off. But I don't know your setup so here is another way to do it:

Change the color of the image to filter out other colors than blue. Feed that image into the glare node and set Mix to 1. This will create an image with only the glare. Add-mix the original image with the glare.